Method for producing a chromium-tungsten coating on tungsten for protection against oxidation at elevated temperatures

ABSTRACT

Method for retarding air oxidation of tungsten by chromium electroplating and high temperature sintering to form an alloy gradient, and free chromium coating whose thickness is controlled by free chromium removal during the sintering and/or by subsequent vacuum firing to provide a crack-free protective coating.

United States Patent TUNGSTEN COATING ON TUNGSTEN FOR PROTECTION AGAINST OXIDATION AT OTHER REFERENCES Lowenheim; F. A., Modern Electroplating, John Wiley &

ELEVATED TEMPERATURES Sons. Inc.. NY. 1963. Second Ed. p. 88, 90, 106,551. 8C|almm80rawlns a Lowenheim; F. A., Modern Electroplating, 1963, 161m 52 us. (:1 2114/37 R WM Page 204/29' Haring; H. E. and Barrows; W. P., Electrodeposition of 51 m C23, 5/52 Chromium from Chromic Acid Baths" Bureau of Standards, 1501 Field oISearch 204/3711, 349Pages436- 5 l 29 Primary Examiner-John H. Mack 5 Assislant ExaminerR. J1 Fay 6] Reerences Cned Attorney-Roland A. Anderson UNITED STATES PATENTS 2,156,262 5/1939 Fink 204/37X 2,555,372 6/1951 Ramage..-

204/37 X BSTRACT: Method for retardlng an oxidation of tungsten 2,805,192 9/1957 Brenner 204/37 by chmmium e'ectmPlafing high empe'awre simering 2 809 127 10/1957 Gibson 204B. UX form an alloy gradient, and free chromium coating whose 10 35 3 1 H1962 y: 204/37 thickness is controlled by free chromium removal during the 3 132 92 5/1964 crooksw 204/37X sintering and/or by subsequent vacuum firing to provide 21 3,198,610 8/1965 Whitfield g 0 41;7 x Crackffrefe 1 l 13 15 2 5 I l I f- I 43 j g/ j 27 l 4 l I 45 l l I l 1 l7 I 1 I I/ Z I I [I 19 I 39 METHOD FOR PRODUCING A CHROMIUM-TUNGSTEN COATING N TUNGSTEN FOR PROTECTION AGAINST OXIDATION AT ELEVATED TEMPERATURES BACKGROUND OF THE INVENTION Tungsten conventionally forms cold junctions for silicongermanium thermoelectric devices wherein the thermal coefficient-of-expansion of the tungsten closely matches that of the silicon-germanium. Heretofore, however, the use of this tungsten in air above 250 has produced a nonprotective, soft and poorly adherent oxide that continues to form until all the tungsten converts to tungsten oxide, thus decreasing the utility of the tungsten as an effective cold junction material. It has been advantageous, therefore, to provide a coating that protects the tungsten from oxidation effects at 600 C., particularly during prelaunch conditions for thermoelectric systems designed to operate in a space vacuum. Also, long term protection has been required in many actual and proposed terrestrial applications, even at temperatures below 600 C.

Heretofore, chromium protective coatings have been applied to tungsten by electroplating, but the chromium has been subject to undesirable cracking, or the chromium has been limited to a thickness of p. or less. Additionally, molybdenum has been protected from oxidation by nickel or aluminum applied over a thin coat of chromium. However, in the intended applications, which involve the use of silicon-germanium thermoelectric materials, the presence of nickel has been intolerable because of its reaction with germanium. Moreover, aluminum is a positive dopant and cannot be used with negatively doped silicon alloys. Its use has also been troublesome near or above its melting point of about 660 C.

SUMMARY OF THE INVENTION This invention was made in the course of, or under a contract with the United States Atomic Energy Commission.

This invention provides a method for retarding air oxidation of tungsten, comprising chromium electroplating the surface of t} tungsten, and high temperature sintering of the electroplated chromium and tungsten to form a chromium-tungsten alloy coating on said tungsten. More particularly, in one embodiment for retarding air oxidation of tungsten for the use for cold junctions for silicon-germanium thermoelectric devices, the chromium electroplating, comprises the steps of applying initial and incrementally increased chromium plating voltages to the tungsten workpiece prepared therefor by cleaning and de-oxidation thereof, and sintering by vacuum firing in helium after the electroplated workpiece is cleaned. In another aspect, the removal of free chromium, provides a crack-free protective coating. With the proper selection of steps and conditions, as described in more detail hereinafter, the desired retarding of air oxidation is achieved.

The above and further novel features of this invention will appear more fully from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are not intended as a definition of the invention but are for the purpose of illustration only.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings where like elements are referenced alike:

FIG. 1 is a photomicrograph at l 100X of the microstructure of a typical alloy coating of this invention;

FIG. 2 is a photomicrograph at I IOOX, after oxidation test, of a two-layered coating, in accordance with this invention, prior to evaporation of residual free chromium by vacuum firing;

FIG. 3 is a photomicrograph at 2630X of the coatings of FIG. 2;

FIG. 4 is a photomicrograph at 72X of unprotected tungsten after oxidation test;

FIG. 5 is a photomicrograph at l lOOX of alloy (only) protected tungsten, in accordance with this invention, after oxidation test;

FIG. 6 is a photomicrograph at llOOX of microcracking after 800 C. hydrogen firing for 10 minutes of a chromium coated tungsten workpiece;

FIG. 7 is aphotomicrograph at l lOOX of a chromium alloy coated tungsten produced by helium firing at I600 C. for 10 minutes, and after relatively mild tungsten etch;

FIG. 8 is a partial cross section and side view of a thermoelectric device wherein the tungsten cold junction thereof is protected against air oxidation by the method of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention is useful in forming cold junctions for (63 percent silicon) silicon-germanium thermoelectric devices, particularly those operating at over 600 C. in air for terrestrial or space applications. However, the method of this invention is also useful for both high and low temperature cold junctions for silicon-germanium thermoelectric devices or any other application where tungsten is subject to oxidation. To this end, the method of this invention protects the tungsten from air oxidation for use in connection with a wide variety of configurations and applications.

In accordance with this invention, the electroplating process uses a standard chromium plating solution, and small, general-purpose equipment. A typical setup, comprises a polypropylene tank, lead anodes located about 3 cm. from the tungsten workpiece, plastic-insulated stainless steel tweezer racks, and a quartz covered immersion heater. One suitable chromium plating solution contains 300 g./l. of CrO and 3 g./l. of H 80, at 54:*:2 C. Generally, the tungsten workpiece is flat in shape, and comprises fully dense tungsten cut from swaged rod. Advantageously, the tweezer rack grips the tungsten workpiece at the edges thereof. The basic procedure is described in the following example.

EXAMPLE I. Sandblast the tungsten surface.

2. Ultrasonically wash the tungsten in organic solvent, typically an acetone-methanol mixture.

3. Rack tungsten for plating.

4. Soak tungsten for 1 minute in a solution of I00 g./l. KOH and 200 g./l. K Fe (CN) at room temperature followed by a thorough rinsing in running water.

5. Apply initial plating voltage to the rack prior to immersion of racks and tungsten in the bath of chromium plating solution, then plate initially at 6 amp./dm.

6. Increase plating current incrementally, reaching 30 amp./dm. after 4 to 5 minutes.

. Plate at 30 amp/dm. to produce average chromium plating thickness of 20 to 25p. after 1 hour and 35 to 40p after minutes.

8. Rinse, unrack and rinse and dry the chromium plated tungsten.

. alloy the chromium and tungsten by firing in helium, with a typical schedule, comprising firing for 10 to 30 minutes at l450 C. and 500 Torr.

l0. Unalloyed chromium is removed by higher temperature at the end of step 9 or by vacuum firing for 2 to 3 minutes at 1450 C.

It will be understood from the above that the process of this invention forms a W-Cr solution alloy coating on the surface of the tungsten workpiece and that the microvisual thickness of this W-Cr alloy coating is a function of the firing time and temperature ranging from 8 to 15p. for the normal process given in the above described example.

A typical microstructure, is shown in FIG. 1. FIGS. 2 and 3, illustrate the two-layered coating that exists prior to removal of the free chromium at 1 450 C. or by vacuum firing.

Chromium deposits, in accordance, with the described method of this invention provide average plating thicknesses of about 2011., although satisfactory chromium plating coatings up to 40p. in thickness have been produced in accordance with the method of this invention. However, no attempt was made to establish any maximum limit for the chromium plating thickness.

Based upon the above-described space application for the process of this invention, oxidation resistance is defined in terms of 336 hours (2 weeks) storage in air and at 600 C. Test results are shown in table I and H68. 2-5.

chromium deposition. As the current density and voltage are increased, the current efficiency, that is, the fraction of the current that reduces chromium ions, also increases. Where low voltage hydrogen cleaning has been omitted or inadequate, and chromium reduction has begun prior to complete deoxidation of the surface of the tungsten workpiece, the material may not adhere and will redissolve by the in steps 1 through 6 of the above-described example constitutes a thorough cleaning and deoxidation of the surface of the tungsten workpiece. Actual experiments showed sandblasting to be unnecessary, but it is included as a matter of good general practice.

.The application of the plating voltage prior to immersion prevents surface oxidation by the chromate bath prior to chromium deposition. Also, scrubbing by nascent hydrogen during the low-current (and low voltage) steps reduces passive films already present. in this regard, when the procedure was experimentally varied by allowing the substrate to oxidize (soak) for 1 minute in the plating solution, prior to the application of the voltage, the result was a thinner, but generally adherent plate. Moreover, when such an oxidation was followed by application of the full plating current (and voltage), the results were still thinner plating, severe cracking in the electroplated chromium coat, and various degrees of peeling from the tungsten base metal. Prior sandblasting reduced, but did not always eliminate, such peeling. The degree of nonadherence and cracking was reduced by lowering the current density.

The importance of the immediate application of voltage is understandable in terms of two competing reactions, one of which operates spontaneously. in this regard, chromic acid oxidizes the surface according to the reaction:

W+Cr O-, +8H** WO +2 Cr" +4H O l. with an E of +1.42 volts. On the other hand, the reduction reaction:

WO +6H*+6e' W+3 H 2.

has an E of 0.09 volt, which is not significant in the absence of an externally applied voltage. Application of the plating voltage, which results in liberation of hydrogen at appreciable overvoltages, causes this reaction to proceed essentially to completion. Adherent plating of chromium takes place only on a conductive, oxide-free tungsten surface. Oxidation of the substrate by reaction (1 is, therefore, expected to reduce the amount of chromium deposition because of the time and cumulative charge flow required before the oxide is removed by reaction (2).

Lack of immediate adherent plating after an initial chromic acid oxidation was confirmed by firing tests involving wet hydrogen (dew point -25 C.) at 800 C. When specimens plated for one minute with immediate voltage application were so fired, a typical chromic oxide color pattern was observed. A l-minute soak" of the tungsten workpiece prior to voltage application followed by 2 minutes of plating" produced a chromium-free surface unaffected by the firing. Chemical analysis of such a sample (sensitivity approx. ug.) was expected on the basis of constant current efficiency.

Low voltage hydrogen plating" in steps 5 and 6 of the above-described example of the process of this invention, is done initially at voltages below the value necessary for whose E is +0.74 volt. in principle, concurrent continuation of reaction (2) ought to terminate in the formation of oxidefree tungsten suitable for adherent electroplating. As cleaning proceeds, however, it is postulated that there is an intermediate situation in which chromium atoms remain in place and grow into a continuous layer, but lack maximum adherence to the tungsten base metal of the tungsten workpiece. This theory would account for the observed result of the process variant, i.e., nonadherent plating, and for the cracks through the entire chromium thickness.

Unlike most plated chromium, the layer produced by steps l-8 of the process of this invention, is nearly, though not totally, free of microcracks in the as-plated condition. However, the mismatch of thermal expansion with tungsten causes considerable microcracking upon heating. For example, the specimen shown in FIG. 6, was hydrogen-fired at 800 C. for 10 minutes. As illustrated in the photomicrograph of this figure, however, the cracks all terminate before reaching the tungsten surface of the tungsten workpiece. Thus under these firing conditions, or the firing conditions of the above example of this invention, the adherence of the chromium to the surface of the tungsten workpiece holds the described surface layer of chromium in place and prevents stress-relieving movement that causes crack propogation.

It will be understood from the above that this invention permits considerable variation in the firing schedules. in this regard, chromium and tungsten have high solid solubility in each other reaching complete mutual solubility at l405 C. In this regard, diffusion coating of the tungsten-chromium alloy is produced by heating the chromium electroplated tungsten workpiece to 1685 C. for 10 minutes to produce an alloy coating thickness of 0.3-0.4 mil. However, it has been discovered in accordance with this invention, that diffusion alloying occurs below that temperature, resulting in the separate alloy phases shown in FIGS. 2 and 3. Thus, for example, heating the chromium plated workpiece to l450 C. for 10 minutes in an inert or reducing atmosphere produces an alloy coating gradient thickness of 0.2-0.3 rnil.

It was also discovered in accordance with this invention that the presence of the inert or reducing atmosphere during thefiring of this invention, slows the competitive process of chromium evaporation, thus to control the chromium thickness and underlying alloying gradient. In this regard, the process of this invention provides for removal of free chromium to a desired thickness by high temperature sintering and/or vacuum firing at the end of the process, wherein the underlying alloy gradient remains undisturbed. For example, a convenient method of removal comprises heating in vacuum to 1400 C. for 5 minutes or heating in vacuum to 1450 C. for 2 to 3 minutes. However, inert or reducing gas heating to 1600' C. after step 9 of the above example may be used alternately.

It has been found that the protected tungsten workpiece made by steps 1-9 of the above-described example can be used for a cold junction for a silicon-germanium thermoelectric device. Such a device is shown in FIG. 8, wherein the elements of device 11, comprise (63% Si) SiGe N and P thermoelements l3 and 15, cold stack 17, mount stud (steel) 19, radiator-base plate (aluminum) 21, nut (steel) 23, hot shoe 25, thermal insulation (Dynaquartz) 27, the chromium-tungsten alloy protected tungsten cold shoe 29 of this invention, pedestal (copper) 31, compensator (tungsten) 33, which may also be protected in accordance with this invention, electrical connector (copper) 35, electrical insulator (alumina) 37 and compensator 39, made of copper or protected tungsten in accordance with this invention. In operation, this device is advantageously operable up to 600 C. in a space vacuum, but may also be used in terrestrial or space applications. Likewise, however, the device 11 may have protected tungsten workpieces made by steps 1-10 of the above described example, wherein the free chromium outer layer is reduced in thickness by a short period of high temperature inert or reducing gas sintering (e.g. at from l,450 to 1,495 C.) or vacuum firing at l,450 C. for minutes. In this case the tungsten base material 41 of the tungsten workpiece, e.g. cold shoe 29, has a chromium-tungsten alloy gradient coating 43, and an outer free chromium coating 4l5 of controlled thickness. W Wm It will be understood from the above, that alloying above l,495 C. eliminates the clear phase boundary as illustrated in FIG. 1, Moreover, with sufficient time and temperature, simultaneous diffusion and vaporization consumes the whole plating volume. The specimens shown in FIGS. 1 and 7, prepared by helium firing at l,600 C. for 10 minutes, illustrates this. However, the application of a tungsten-reactive etchant used relatively sparingly in preparing the sample of FIG. 7, illustrates that an alloy concentration gradient remains.

While the alloy firing of this invention produces wider cracks in the free chromium that at 800 C., the number of cracks reduces since the former results in competition between the tendency to crack under differential expansion stresses, and stress relief by annealing. Also, none of the cracks enter the volume originally occupied by the tungsten.

The coating system of this invention, has the advantage of protecting the tungsten workpiece under the test conditions described in table I, and/or in preventing oxide accumulation at fissure bottoms. Moreover, the described chromium removal step of this invention, at the end of the process thereof, by either one or two step firing, prevents cracking of the alloy layer all together, since material movement, believed to be by diffusion and/or transient vaporization, provides crack healing. Thus, the crack-free alloying process of this invention greatly retards air oxidation of the tungsten workpiece for the protection of cold junctions for silicon-germanium thermoelectric elements, or any other application employing tungsten in an oxidizing atmosphere.

What is claimed is:

1. Method for preparing tungsten thermoelectric junctions for transmitting electrical power from silicon-germanium thermoelectric devices in air at 600 C., comprising mechanically abrading and cleaning a dense tungsten thermoelectric junction cut from swaged rod, deoxidizing the surface of said tungsten in a chromium electroplating solution by preapplying plating voltages and currents that prevent the oxidation and the initial plating of said tungsten in said solution while selectively producing therein nascent hydrogen in sufficient quantities for a sufficient period of time to scrub said tungsten with said hydrogen for effecting the deoxidation of the oxide occurring on said tungsten after said mechanical abrading and cleaning, increasing said plating voltages and currents sequentially, slowly, and variably after said deoxidizing for effecting the in place production ofchromium atoms from said solution and the adherent deposition thereof in a continuous plating layer from about to about 401.4. thick on said tungsten, cleaning said chromium plated tungsten for alloying said tungsten with said chromium plating at high temperatures wherein cracks appear in said chromium plating due to the differential thermal expansion between said tungsten and said chromium plating thereon, and alloying a portion of said chromium plating into said tungsten at 1,450 C. in an inert gas at 500 Torr pressure to form and diffuse said alloy into said cracks in said chromium on said tungsten in a sandwich in accordance with the self-healing production of said alloy below said continuous chromium layer and the holding of said chromium in said layer in place by said alloy to prevent stress-relieving movement that produces the propagation of said cracks from said chromium through said alloy to said tungsten, whereby said junction transmits said electrical power without oxidation and volatilization of said tungsten for long periods of time in air at up to 600 C.

2. The invention of claim 1 in which said electroplating and sintering produces a two-layered coating, comprising a chromium-tungsten alloy coating underneath a chromium coating, the thickness of said chromium-tungsten alloy coating is initially from 8-15p. thick, and said chromium coating is decreased in thickness by vacuum firing at a temperature of about l,450 C.

3. The invention of claim 1 in which said chromium electroplating, comprises the steps of:

a. sandblasting the surface of said tungsten;

b. ultrasonicall washing said tungsten in organic solvent; c. soaking sai tungsten for 1 minute in a solution of 100 g./l. KOI-l and 200 g./l. K Fe (CN) at room temperature;

d. rinsing said tungsten;

e. applying an initial chromium plating voltage to said tungsten in a chromium plating bath at 6 amp./dm.

f. increasing said chromium plating voltage incrementally in steps up to 30 amp./dm. after 4 to 5 minutes;

g. chromium plating said tungsten at 30 amp./dm. to produce a chromium plated surface on said tungsten of from 20 to 25p. after 1 hour and from 35 to 40p. after minutes;

h. rinsing and drying said chromium plated tungsten; and

i. alloying said chromium and tungsten at the surface of said tungsten by firing in helium at 1,450" and 500 Torr.

4. The invention of claim 1 in which said hydrogen is liberated from an electroplating solution, comprising chromic acid and sulfuric acid, wherein there is a spontaneous oxidation reaction according to the formula:

W+Cr 0 +8H*-* WO +2Cr* +4 H 0, where the reaction has an E of +1 .42 volts; and a competing reduction reaction in the presence of a plating voltage according to the formula:

WO;,+6I-l*+6e W+3 H 0 where the reaction has an E of 0.09 volt.

5. The invention of claim 4 in which the initial electroplating voltage is below the value necessary for chromium deposition, and the reaction proceeds substantially to completion by the application of sufficient electroplating overvoltages.

6. The coated tungsten produced by the method of claim I for said use as a cold junction for said silicon-germanium thermoelectric device.

7. The method of claim 1 in which said silicon-germanium thermoelectric device is contacted with said chromium coating of said alloy coated tungsten in air and heated to 600 C. for conducting electrical energy from said thermoelectric device through said chromium coating, said alloy and said tungsten, whereby said alloy restrains said chromium from producing cracks through said alloy to said tungsten through which said tungsten could be oxidized, thereby to prevent the volatization of said tungsten through said cracks.

8. The invention of claim 1 in which said chromium is removed to expose said alloy, said alloy is contacted with said silicon-germanium device in air, and said alloy coated tungsten and thermoelectric device are heated in said air to 600 C. while in contact with each other, whereby said alloy resists the formation of cracks therein due to the differential thermal expansion between said tungsten, said alloy, and the silicongermanium of said thermoelectric device.

l l l i 

2. The invention of claim 1 in which said electroplating and sintering produces a two-layered coating, comprising a chromium-tungsten alloy coating underneath a chromium coating, the thickness of said chromium-tungsten alloy coating is initially from 8- 15 Mu thick, and said chromium coating is decreased in thickness by vacuum firing at a temperature of about 1450* C.
 3. The invention of claim 1 in which said chromium electroplating, comprises the steps of: a. sandblasting the surface of said tungsten; b. ultrasonically washing said tungsten in organic solvent; c. soaking said tungsten for 1 minute in a solution of 100 g./l. KOH and 200 g./l. K3Fe (CN)6 at room temperature; d. rinsing said tungsten; e. applying an initial chromium plating voltage to said tungsten in a chromium plating bath at 6 amp/dm.2; f. increasing said chromium plating voltage incrementally in steps up to 30 amp/dm.2 after 4 to 5 minutes; g. chromium plating said tungsten at 30 amp/dm.2 to produce a chromium plated surface on said tungsten of from 20 to 25 Mu after 1 hour and from 35 to 40 Mu after 90 minutes; h. rinsing and drying said chromium plated tungsten; and i. alloying said chromium and tungsten at the surface of said tungsten by firing in helium at 1450* and 500 Torr.
 4. The invention of claim 1 In which said hydrogen is liberated from an electroplating solution, comprising chromic acid and sulfuric acid, wherein there is a spontaneous oxidation reaction according to the formula: W+ Cr207 2+ 8H WO3+ 2Cr 3+ 4H20, where the reaction has an E* of +1.42 volts; and a competing reduction reaction in the presence of a plating voltage according to the formula: WO3+ 6H +6e W+ 3 H20 where the reaction has an E* of -0.09 volt.
 5. The invention of claim 4 in which the initial electroplating voltage is below the value necessary for chromium deposition, and the reaction proceeds substantially to completion by the application of sufficient electroplating overvoltages.
 6. The coated tungsten produced by the method of claim 1 for said use as a cold junction for said silicon-germanium thermoelectric device.
 7. The method of claim 1 in which said silicon-germanium thermoelectric device is contacted with said chromium coating of said alloy coated tungsten in air and heated to 600* C. for conducting electrical energy from said thermoelectric device through said chromium coating, said alloy and said tungsten, whereby said alloy restrains said chromium from producing cracks through said alloy to said tungsten through which said tungsten could be oxidized, thereby to prevent the volatization of said tungsten through said cracks.
 8. The invention of claim 1 in which said chromium is removed to expose said alloy, said alloy is contacted with said silicon-germanium device in air, and said alloy coated tungsten and thermoelectric device are heated in said air to 600* C. while in contact with each other, whereby said alloy resists the formation of cracks therein due to the differential thermal expansion between said tungsten, said alloy, and the silicon-germanium of said thermoelectric device. 